Ongoing studies of the synaptic circuitry of the pulvinar/LP complex suggest that the three main subdivisions of the cat pulvinar/LP complex are each uniquely organized, and three potential "loops" that connect each subdivision with distinct cortical and subcortical regions have been identified. The current proposal tests for the existence of these loops, and examines certain features of the morphology, synaptic arrangements, and the neurotransmitters and receptors used in the pathways that comprise the predicted loops. The proposed studies will examine 1) the synaptic connections made between cortical area 17, the lateral LP nucleus, and the medial bank of the posterior lateral suprasylvian sulcus (area PMLS), 2) the synaptic connections made between the lateral bank of the posterior lateral suprasylvian sulcus (area PLLS), the superior colliculus, and the medial LP nucleus, and 3) the synaptic connections made between cortical area 7, the pretectum, and the pulvinar nucleus. Combinations of neuroanatomical tract tracing, immunocytochemistry, and electron microscopy will be used to examine the synaptic organization of cortical and brainstem inputs within the pulvinar/LP complex, pretectum and superior colliculus. In addition, the physiological effects of substance P, the neurotransmitter used in the projection from the superior colliculus to the medial LP nucleus will be examined by recording from brain slices maintain in vitro.